By means of the strengthening of automotive exhaust-gas regulations, the developments of catalysts for converting exhaust gas have advanced, and thereby the emission amounts of HC, CO and NOx in exhaust gases have been becoming extremely less recently. As for these catalysts for converting exhaust gas, the following are representative ones: three-way catalysts that are used at air-fuel ratios that are controlled in the vicinity of stoichiometry; and NOx storage-and-reduction catalysts that are used in atmospheres that are turned into rich atmosphere during lean atmosphere intermittently.
However, since these catalysts for converting exhaust gas are those in which Pt, Rh and the like are adapted into the active metals, the catalytic activities are not generally expressed until they reach the activation temperatures of 200° C. or more, and they are associated with such a problem that the harmful components have been emitted without ever being converted in low-temperature region such as at the time of starting.
Hence, the following have been carried out: an adsorption material, such as zeolite, is put in place on an upstream side of a catalyst for converting exhaust gas; the harmful components are captured onto the adsorption material in low-temperature region; and the harmful components are eliminated from the adsorption material after the catalyst for purifying exhaust gas has undergone temperature rise to the activation temperature or more, and then they are converted with a downstream-side catalyst for converting exhaust gas.
For example, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2000-312,827, there is set forth a catalyst for converting exhaust gas, catalyst in which a front-stage catalyst with Rh being supported on a zeolite is put in place on an exhaust-gas upstream side, and a rear-stage catalyst with Pt or Pd being supported is put in place on a downstream side thereof. In accordance with this catalyst for converting exhaust gas, NOx adsorb onto the front-stage catalyst in low-temperature region, and NOx, which have been eliminated from the front-stage catalyst in high-temperature region, are then converted by means of reduction at the rear-stage catalyst.
However, in low-temperature NOx adsorption materials that utilize zeolites, such as the front-stage catalyst as set forth in the aforementioned gazette, there have been such a phenomenon that, due to the influence of moisture that is included in exhaust gases, the NOx adsorption amount lowers. Hence, in Japanese Patent Publication Gazette No. 3,636,116, there is proposed an exhaust converter in which a moisture trap is disposed on an upstream side of a low-temperature NOx adsorption material. According to that gazette, the moisture content in exhaust gases that flow into the low-temperature NOx adsorption material is controlled to 0.4%-2.4% approximately by means of the moisture trap; and accordingly NOx adsorption amount of the low-temperature NOx adsorption material augments; and thereby it is possible to reduce the release of unconverted NOx considerably at the time of low temperatures.
According to the studies by the present-application inventors, it became apparent that the lowering degree of NOx adsorption amount by means of moisture differs depending on the types of NOx adsorption material. And, it was found that NOx adsorption materials comprising zeolites in which transition metal ions are supported by means of ion exchange adsorb NOx in an extremely great amount in low-temperature region; however, it became apparent that, in such NOx adsorption materials, the less the moisture content in exhaust gas that flows in is the more the NOx adsorption amount augments.
In Japanese Patent Publication Gazette No. 3,636,116, although silica gel, activated carbon, alumina, type A zeolite, and the like, are exemplified as the moisture trap, these moisture traps are not practical because it is difficult to set the moisture contents in exhaust gases that flow into the NOx adsorption material to 0.4%-2.4% or less, or because they are associated with such a drawback that the hydrothermal resistance is low, though it is feasible for them to trap moisture sufficiently.
The present invention is one which has been done in view of the aforementioned circumstances, and it is an assignment to be solved to make an exhaust-gas converting apparatus that can adsorb a great amount of NOx in low-temperature region by using an NOx adsorption material comprising a zeolite that includes a transition metal ion and then selecting a moisture adsorption material that is optimum for it.